A wireless sensor network may in general be defined as a distributed system consisting of many sensor nodes each equipped with a wireless radio transceiver along with application-specific sensors and signal processing hardware. Due to the typically short range of low-power RF transceivers, communication between nodes takes place via multi-hop through neighboring nodes.
The resources that can be put on a node are generally limited by the size and cost constraints, and the requirements such as flexibility and ease of installation. Therefore the nodes are typically resource-constrained while the network throughout its required lifetime should operate reliably. Therefore, energy-efficiency and guaranteed low delays are two primary goals in the design of a sensor network protocol.
Many prior studies have indicated that idle listening is a major source of energy waste in network nodes (see one or more of: Wei Ye, John Heidmann, and Deborah Estrin, “An Energy Efficient MAC Protocol for Wireless Sensor Networks,” IEEE Infocom 2002; Wei Ye, John Heidmann, and Deborah Estrin, “Medium Access Control with Coordinated Adaptive Sleeping for Wireless Sensor Networks,” ACM/IEEE Transaction on Networking, Vol. 12, No. 3, pp. 493-506, June 2004; Tijs van Dam and Koen Langendoen, “An Adaptive Energy-efficient MAC Protocol for Wireless Sensor Networks,” Proceedings of the 1st International Conference on Embedded Networked Sensor Systems (SenSys '03), 2003, pp. 171-180; and Gang Lu, Bhaskar Krishnamachari and Cauligi Raghavendra, “An Adaptive Energy-Efficient and Low-Latency MAC for Data Gathering in Wireless Sensor Networks,” IEEE Proceedings of the 18th International Parallel and Distributed Processing Symposium (IPDPS04)). For many radio chips, the energy used for idle listening is almost comparable with energy used for reception or transmission. Therefore to increase the lifetime, it is desirable to put the nodes in low power or sleep mode when there is no activity in the network.
Significant energy savings can be realized by putting the nodes into sleep mode and periodically awakening them and checking the channel for any signal (sniffing the channel). However, this can also increase the message delivery latency because for example information of a measurement by a sensor or a detected event can only be sent when the receiving node is active. Also, delivery latency can be increased when the message passes through multiple hops, an intermediate node needs to wait for the node in the next hop to wake up.
Researchers in ad hoc and sensor networks continue to search for new wakeup techniques to save power without suffering the large latency penalties associated with the wakeup process. Current methods can be divided into two main categories: scheduled wakeups, and wakeup on-demand (out of band wakeup).
Using scheduled wakeups, the nodes follow deterministic (or possibly random) wakeup patterns. Time synchronization among the nodes in the network is generally assumed. However, asynchronous wakeup mechanisms which do not require synchronization among the different nodes are also categorized in this class. Although asynchronous methods are simpler to implement, they are not as efficient as synchronous schemes, and in the worst case the delay can be very long.
When using wakeup on-demand methods, it is assumed that the nodes can be signaled and awakened at any point of time. This is generally accomplished by having two wireless interfaces or radios. A first radio is used for data communication and is triggered by the second ultra low-power radio which is used only for paging and signaling. Although these methods can be optimal in terms of both delay and energy, the cost issues, currently limited available hardware options and stringent system requirements prohibit the design of such systems. Consequently, there is a need for scheduled wakeup methods under which wakeups of sensor nodes are scheduled efficiently such that urgent but rare messages can be transmitted with small guaranteed end-to-end delay through multiple hops in the network.